1. Field of the Invention
This invention relates to driving devices for electro-luminescence units, and more particularly to driving devices for electro-luminescence units in instruments operating with a electrical power source of low voltage such as battery.
2. Description of the Related Art
To produce electro-luminescence of high intensity, an AC voltage of more than several tens of volts must be applied to an electro-luminescence unit. For the instrument equipped only with an electrical power source of low voltage (several volts) such as the dry battery, it has, therefore, been a general practice in the prior art to make use of a DC-AC converter as the means for producing an alternating increased voltage by which the electro-luminescence unit is driven.
FIG. 5 illustrates a conventional example of the driving device for electro-luminescence units whose boosting circuit employs such a DC-AC converter.
In FIG. 5, so long as a control unit 6 renders non-conducting a transistor 5, a transistor 2 having associated resistors 3 and 4 is non-conducting so that a DC-AC converter circuit is in a inoperative state.
When the control unit 6 changes the transistor 5 to the conducting state, the transistor 2 is also rendered conductive and battery 1 is connected with the DC-AC converter circuit, initiating operation thereof.
As the electrical power is supplied to the DC-AC converter circuit, current flows through a resistor 8 into the base of a transistor 7. When the transistor 7 starts to transit from the non-conducting to the conducting state, current flows into a tap, b, of a boosting transformer 10 through the collector-emitter path of the transistor 7. Thereby, a positive voltage is caused to appear at another tap, c, of the boosting transformer 10. Therefore, current flows through a capacitor 9 to the base of the transistor 7 and the transistor 7 transits to progressively more conductive states.
In a certain time from the start of transition of the transistor 7 from the non-conductive to the conductive state, the transistor 7 reaches a full conductive state. From this time onward, the voltage applied to the tap, b, of the boosting transformer 10 does not increase. Therefore, at this point in time, the voltage produced at the tap, c, of the boosting transformer 10 starts to decrease. This causes flow of current to the base of the transistor 7 through the capacitor 9 in a direction to render the transistor 7 less conductive. The decreasing of the voltage produced at the tap, c, of the boosting transformer 10 proceeds further and, the transistor 7 reaches the totally non-conductive state. Such procedure then repeats itself.
By the recycling of turning on and off of the transistor 7, the voltage at the tap, b, of the boosting transformer 10 is changed intermittently. This changing is reflected in a boosted AC voltage across the ends, a and d, of the winding of the boosting transformer 10. Using this voltage, the electro-luminescence unit 11 is driven.
As is seen in FIG. 5, the above-described type of electro-luminescence driving circuit using such a DC-AC converter has numerous parts. Also, because of its using the boosting transformer, it is difficult to minimize the size and weight of the device. In application to portable instruments using dry batteries as the electrical power source, therefore, shortcomings existed in that the portability was reduced, and a high production cost was required.